Okay so people are confused because of the gang. Marinette is not part of the gang! She is just friends with them, and occasionally hangs out with them. Think of it like this,
Mia, Luci's best friend, owns a shop which Marinette occasionally visited before she got akumatized. After a while, they became friends.
Marinette is also friends with Victoria, who runs Café Miraculous.
Once Marinette got hit by the akumatization affect, she started to become more violent and picked more fights, or wanted to pick more fights. Lila picked a fight with her, so she went to Mia's shop to see if she can get some equipment, when she saw Luci's hoodie.
This is where a lot of people get confused!
Mia explained that that hoodie is her old best friend's hoodie when coincidentally, Luci walks into the store and Mia recognises her. Luci recognises her hoodie, and Blah Blah Blah they reunited with all of their old friends. Star, Victoria and Luci had a fight because of Emily "death", and Mia moved just after Luci jumped.
So, Lila hires Alex and his gang to help her fight Marinette, because she knows that she can't beat her. What Lila doesn't know is that Alex is scared of Luci, because she broke his nose when they were younger and would have killed him if Mia didn't stop her.
Luci, her friends and her gang tag along, because of Alex and his gang. They know how dangerous Alex is, and Luci is pretty protective of her friends. I guess as a thanks, they help Marinette.
I can't remember if I added this into the chapter or not, but Luci told Marinette that she could text her if she needed help. Kagami and Chloe find Marinette and take her back to her house, to which her parents start to get informed of what has been happening in Marinette's life.
Mia asks if Marinette wants to go to their high school reunion, 'cause why not. Alex invites Lila 'cause why not and Alex and Star propose to their partners.
ANWAY, after Luci and Felix, her brother, find out that Jonathan, a member from Alex's gang and Victoria are their kidnapped siblings, Luci's dad says "former lighting gang". The gang doesn't exist anyway more.
I get that this doesn't make sense, but please bear with me, these characters are important later in the story, especially towards Marinette's health. Hopefully this clears it up a little.